In U.S. Pat. No. 4,278,514 I disclosed an electroplating solution for the deposition of palladium containing said palladium in the form of a soluble organopalladium complex formed from an inorganic palladium salt and an organic polyamine complexing agent having from 2 to about 8 carbon atoms and from 2 to about 5 amino groups; said solution having a pH from about 3-7, and said solution containing about 1-50 grams per liter of a cyclic organic imide of formula ##STR1## in which each R is independently selected from the group consisting of hydrogen, alkyl of 1-5 carbon atoms, or alkoxy of 1-5 carbon atoms. I also disclosed that in the electroplating solution hereinabove described, a further improvement in brightness of the deposit could be obtained by including in the solution a quantity of about 1-50 grams per liter of organic polyamine complexing agent beyond that forming a part of the soluble organopalladium complex.
Physical, mechanical and electrical properties of palladium electrodeposits from solutions as described in U.S. Pat. No. 4,278,514 have been measured and are published in the literature [R. J. Morrissey, Plating and Surface Finishing, 67, 44 (Dec. 1980)]. The deposits are subject to microcracking, the extent of which can be reduced by increasing the temperature of the plating solution. This, however, causes a concurrent loss of deposit brightness. X-ray diffraction studies show the deposits to be strongly (110) oriented under most plating conditions. [Note: "110" and "111" refer to Miller Indices. B. D. Cullity, Elements of X-ray Diffractions, Addison-Wesley, Mass. 1956].
Although the occurrence of microcracking is quite common in palladium electrodeposits, it is in general considered to be undesirable. It may also be noted that the atomic arrangement in metallic palladium is face-centered cubic. The slip plane, that is, the crystallographic plane in which the atomic layers of a crystal can slide over each other with minimum friction, is the (111) plane in face-centered cubic crystals. Deposits oriented with the slip plane parallel to the substrate surface are generally superior in resistance to sliding friction and abrasive wear than those otherwise oriented. On this basis one would expect that, other things such as deposit hardness being equal, the resistance to abrasive wear of a (110) oriented palladium electrodeposit such as is ordinarily deposited from electroplating solutions as described in U.S. Pat. No. 4,278,514 would be inferior to that of a (111) oriented deposit.
The foregoing comments notwithstanding, many aspects of the chemistry of electroplating solutions for palladium as described in U.S. Pat. No. 4,278,514, specifically freedom from added ammonium ion, a pH range from about 3-7, and the use of a soluble organopalladium complex as the source of palladium, remain highly desirable in that they permit the deposition of palladium directly onto substrates including nickel, copper, and alloys thereof without requiring the use of an intermediate strike coating.
Accordingly, it is an object of this invention to provide a formulation for the electroplating of palladium, said formulation being free of added ammonium ion, and of a pH range so as to be applicable for plating palladium deposits onto substrates including nickel, copper, and alloys thereof without requiring the application of a prior strike; and said formulation being suitable for plating palladium deposits which are free of microcracking, of very high brightness, and of (111) preferred crystal orientation. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.